Introduction to the Complementary Fired Combined Cycle Power Plant

John H. Copen (Presenter of Record) Siemens Power Generation
Terrence B. Sullivan Siemens Power Generation

This paper challenges the conventional method of fuel-based bottoming cycle power augmentation in a combined cycle plant, in which a fuel source is combusted in the hot flue gas stream internal to a combined cycle HRSG – also known as supplementary firing or duct firing. Although duct firing is an effective means of increasing plant capacity, it significantly reduces the plant efficiency. Additionally, as the world fuel markets continue to incur a substantial increase in demand, power plant owners and operators are more actively seeking plant solutions that provide better performance flexibility.

To provide a solution that would allow plant owners better dispatch options, a system was developed that provides base load outputs with maximum efficiencies as well as incrementally selectable peaking outputs with high plant efficiencies. Termed as Complementary Fired Combined Cycle (CFCC), this system is predicated on the use of fractionally sized gas turbines, with their exhaust ducted into the HRSG(s) associated with their base GT(s). This configuration offers very high peak loading efficiency as well as the possibility to increase the level of power augmentation due to its unique impact on the HRSG. This system can be applied to new unit construction, and also has the potential to be retrofitted into plants with and without existing duct firing systems.

This paper explains the Complementary Fired Combined Cycle plant design concept and compares its plant performance characteristics with conventional duct fired plants. Retrofitting applications are also explored. Ancillary advantages of the CFCC plant are renumerated, along with economic comparisons of plant Life Cycle Costs.

Traditional methods for combined cycle peak loading, although effective for providing power, are not well suited to the current global energy and economic models in which higher peak plant efficiency is steadily becoming a critical design criteria. For this reason, a novel concept for providing enhanced combined cycle performance, both on a power and on a heat rate basis, was developed. This document describes this system and provides specific performance calculations for the application of currently available equipment.

This system can be applied to new unit construction or as a retrofit to already existing power plants as a power peaking application or a heat rate reduction option. This art departs from the traditional concept of combusting a supplemental fuel directly in the path of the flue gas in the HRSG(s), as detailed in U.S. Patent # 6,606,848, “High power density combined cycle power plant system” and instead uses high efficiency industrial-sized gas turbines and generator sets (shaft power being converted to electrical energy through electrical generators and being delivered to grid) as a combusting venue while using the same HRSG(s) of the base combined cycle power plant to recover the rejected heat of the industrial GT(s). This concept fits particularly well when considered and used for high ambient temperature peaking applications since the typical HRSG design basis is a cold day application when the largest flue gas mass flows are achieved. As ambient temperatures increase, the base GT exhaust mass flow(s) decrease, thereby allowing ample flue gas mass flow augmentation capacity.

The general concept of this system is the introduction of additional energy to the flue gas path of a GT / HRSG set by introduction of the waste heat of an industrial sized gas turbine. In this system, referred to as Complementary Firing, additional fuel to augment the bottoming cycle output is first combusted in the complementary industrial sized gas turbine. The exhaust of the complementary turbine is then mixed into the base GT flue gas path in the HRSG. This differs from the conventional peak loading scheme, in which fuel is combusted directly in the HRSG.
Configuration The conceptual design basis for the Complementary Firing system (Figure 1) entails a power plant consisting of:

  • At least one base gas turbine (topping or Brayton cycle) with at least one HRSG and at least one steam turbine, (bottoming or Rankine cycle).
  • At least one industrial or complementary gas turbine and generator set, also called the complementary topping cycle or complementary Brayton cycle.

Design Concept for Complementary Firing System

The multiple Interface Points shown in the above Figure represent the possible tie-in
points that merited evaluation.

Interface Points
In order to optimize the plant performance in complementary fired mode, several possible HRSG interface points where the complementary topping cycle exhaust gas mixed with the base exhaust gas were studied. As can be seen in Figure 1, three possible insertion points were considered….<…>

To read this paper Download PDF from Siemens

Leave a Reply